Various approaches have been proposed for monitoring the locations of objects within an area. One such approach uses radio frequency (RF) transmitters or tags, attached to the objects to be tracked, and an array of receivers for receiving tag transmissions throughout a tracking area. The signal strengths of the received tag transmissions may then be used to determine a tag's distance from the receivers. One problem with this approach is that it is not well suited for monitoring the locations of objects within a building having multiple rooms. In such an environment, the unpredictable attenuation of transmissions passing through walls and other structures, and the effects of multi-path distortion, make signal strength only marginally useful as a parameter for determining distance.
One solution to this problem, proposed in U.S. Pat. No. 5,119,104 to Heller, involves accurately measuring the time of arrival of the tag transmission at each of multiple receivers. This time of arrival data is then used in combination to determine the location of the tag. A significant problem with this approach is that a high degree of synchronization is required between the receivers to produce accurate results. As a result, such a system tends to be difficult and expensive to implement.